Wind Power Generator System and Control Method of the Same

ABSTRACT

A wind power generator system according to the present invention includes: a windmill rotor including a blade having a variable pitch angle; a generator driven by the windmill rotor; and a control unit controlling the output power of the generator and the pitch angle of the blade in response to the rotational speed (ω) of the windmill rotor or the generator. The control unit performs a first control in which the output power is controlled in accordance with a predetermined power-rotational speed curve until the rotational speed (ω) is increased to reach a predetermined rated rotational speed, and performs a second control in which the output power is controlled to a predetermined rated power when the rotational speed (ω) exceeds the rated rotational speed; the control unit is responsive to the pitch angle for maintaining a state of performing the second control is or for switching to a state of performing the first control, when the rotational speed (ω) is reduced below the rated rotational speed after the control unit is once placed into the state of performing the second control. This provides a wind power generator system which suppresses output power fluctuation and generation efficiency reduction when a transient wind null occurs.

TECHNICAL FIELD

The present invention relates to a wind power generator system andmethod for controlling the same, particularly, to control of the outputpower and the pitch angle of a wind power generator system adopting avariable-speed and variable-pitch control method.

BACKGROUND ART

One of the promising control methods for a wind power generator systemis a variable-speed and variable-pitch control method, in which therotational speed of the windmill rotor (that is, the rotational speed ofthe generator) is variable and the pitch angle of the blades isvariable. Advantages of the variable-speed and variable-pitch controlmethod include increased energy capture from the wind and decreasedoutput fluctuation.

With respect to the variable-speed and variable-pitch control method, itis of importance to optimize the control of the output power of thegenerator and the pitch angle of the blades. Japanese Translation ofInternational Publication No. 2001-512804 discloses a control method inwhich the torque of the generator is controlled with a field orientationcontrol while the pitch angle is controlled independently of the torqueof the generator. In the disclosed control method, desired output powerof the generator is determined in response to the rotational speed ofthe generator using a lookup table, and a torque command for thegenerator is determined from the desired output power. The torque of thegenerator is controlled by a field orientation control in response tothe torque command. On the other hand, the pitch angle of the blades iscontrolled by PID control, PI control or PD control responsive to thedeviation between the rotational speed of the generator and a desiredrotational speed.

One issue for the wind power generator system is how to deal withoccurrence of a transient wind null, that is, a short-time reduction inthe wind speed. Generally, a wind power generator system is designed togenerate rated power in the case where the rotational speed of thewindmill rotor is equal to or larger than a rated rotational speed. Insuch a wind power generator system, the output power is reduced belowthe rated power, when the rotational speed of the windmill rotor isreduced below the rated rotational speed due to the occurrence of thetransient wind null. This causes output power fluctuation and generationefficiency reduction.

DISCLOSURE OF INVENTION

Therefore, an object of the present invention is to provide a wind powergenerator system which suppresses output power fluctuation andgeneration efficiency reduction even when a transient wind null occurs.

A wind power generator system according to the present inventionincludes: a windmill rotor including a blade having a variable pitchangle; a generator driven by the windmill rotor; and a control unitcontrolling the output power of the generator and the pitch angle of theblade in response to the rotational speed (ω) of the windmill rotor orthe generator. The control unit performs a first control in which theoutput power is controlled in accordance with a predeterminedpower-rotational speed curve until the rotational speed (ω) increasesand reaches a predetermined rated rotational speed, and performs asecond control in which the output power is controlled to apredetermined rated power when the rotational speed (ω) exceeds therated rotational speed; the control unit is responsive to the pitchangle for maintaining a state of performing the second control is or forswitching to a state of performing the first control, when therotational speed (ω) is reduced below the rated rotational speed afterthe control unit is once placed into the state of performing the secondcontrol. Here, the pitch angle is an angle formed between a chord of theblade and a rotation plane of the windmill rotor. Namely, the windmillrotor extracts more energy from wind when the pitch angle is small, andthe windmill rotor extracts less energy from the wind when the pitchangle is large.

The wind power generator system configured as stated above can suppressthe output power fluctuation by using the rotational energy of thewindmill rotor when the wind speed is reduced only for a short time.This is because the wind power generator system according to the presentinvention keeps the output power at the predetermined rated power inresponse to the pitch angle of the blade, when said rotational speed (ω)is reduced below said rated rotational speed. When it is determined fromthe pitch angle of the blade that the system is in a state in which theoutput power can be kept at the predetermined rated power, the outputpower is kept at the rated power, and this allows effectively extractingthe rotational energy of the windmill rotor and suppressing the outputpower fluctuation and the power generation efficiency reduction.

Preferably, when the rotational speed (ω) is reduced below the ratedrotational speed after the control unit is once placed into the state ofperforming the second control, the control unit maintains the state ofperforming said second control for the case when the pitch angle islarger than a predetermined pitch angle, not switching to the state ofperforming the first control until the pitch angle reaches thepredetermined pitch angle.

Preferably, the control unit controls said pitch angle in response tothe difference between the rotational speed (ω) of the windmill rotor orthe generator and the predetermined rated rotational speed and thedifference between the output power and the rated power. In this case,the control unit preferably controls the pitch angle to be reduced whenthe output power is lower than the rated power. The control unitpreferably increases the output power of the generator in response tosaid rotational speed (ω) when a gust is detected.

In the case where the wind power generator system further includes: arotation mechanism rotating a direction of the rotational surface of thewindmill rotor; and a wind direction detector detecting a windwarddirection and the windmill rotor includes a pitch drive mechanismdriving the blade, it is preferable that the control unit controls therotation mechanism so as to move the rotation plane of the windmillrotor away from the windward direction when detecting a failure in thepitch drive mechanism.

Preferably, the control unit controls reactive power outputted from thegenerator to a power grid connected to the generator in response to avoltage of the power grid, and controls the pitch angle in response tothe reactive power.

In the case where the wind power generator system further includes anemergency battery and a battery charger charging the emergency batterywith power received from the power grid, wherein the windmill rotorincludes a pitch drive mechanism driving the blade and wherein theemergency battery supplies power to the pitch drive mechanism and thecontrol unit when the voltage of the power grid connected to thegenerator is reduced, the control unit preferably controls the outputpower to be increased while the emergency battery is being charged.

A method of controlling a wind power generator system according to thepresent invention is a method of controlling a wind power generatorsystem provided with: a windmill rotor including a blade having avariable pitch angle; and a generator driven by the windmill rotor. Thecontrol method includes a control step of controlling output power ofthe generator and a pitch angle of the blade in response to therotational speed (ω) of the windmill rotor or the generator. Saidcontrol step includes steps of: (A) performing the first control inwhich said output power is controlled in accordance with to apredetermined power-rotational speed curve until said rotational speed(ω) increases to reach a predetermined rated rotational speed; (B)performing a second control in which said output power is controlled toa predetermined rated power when said rotational speed (ω) exceeds therated rotational speed; and (C) in response to the pitch angle,maintaining the state of performing said second control or switching tothe state of performing said first control, when said rotational speed(ω) is reduced below the rated rotational speed after the state ofperforming said second control is once established.

The present invention provides the wind power generator system which cansuppress the output power fluctuation and the generation efficiencyreduction even when a transient wind null occurs.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a side view showing the configuration of a wind powergenerator system in one embodiment of the present invention;

FIG. 2 is a block diagram showing the configuration of a pitch drivemechanism of the wind power generator system of the present embodiment;

FIG. 3 is a block diagram showing the configuration of the wind powergenerator system of the present embodiment;

FIG. 4 is a graph showing a power control method performed by the windpower generator system of the present embodiment;

FIG. 5 is a block diagram showing an example of the configuration of amain control unit of the wind power generator system of the presentembodiment;

FIG. 6 is a table explaining operations performed by a power controllerand a pitch controller of the wind power generator system of the presentembodiment;

FIG. 7 is a graph showing an example of an operation performed by thewind power generator system of the present embodiment;

FIG. 8 is a block diagram showing another configuration of the windpower generator system of the present embodiment;

FIG. 9 is a flowchart of a preferred control performed by the wind powergenerator system of the present embodiment;

FIG. 10 is a flowchart of another preferred control performed by thewind power generator system of the present embodiment;

FIG. 11 is a flowchart of still another preferred control performed bythe wind power generator system of the present embodiment; and

FIG. 12 is a flowchart of still another preferred control performed bythe wind power generator system of the present embodiment.

BEST MODES FOR CARRYING OUT THE INVENTION

A wind power generator system according to the present invention will bedescribed hereinafter in detail with reference to the attached drawings.

FIG. 1 is a side view showing the configuration of a wind powergenerator system 1 in one embodiment of the present invention. The windpower generator system 1 is provided with a tower 2 and a nacelle 3provided on the top end of the tower 2. The nacelle 3 is rotatable inthe yaw direction and directed to a desired direction by a nacellerotation mechanism 4. Mounted in the nacelle 3 are a wound-rotorinduction generator 5 and a gear 6. The rotor of the wound-rotorinduction generator 5 is connected to a windmill rotor 7 through thegear 6. The nacelle 3 additionally includes an anemometer 10. Theanemometer 10 measures the wind speed and the wind direction. Asdescribed later, the nacelle 3 is rotated in response to the wind speedand the wind direction measured by the anemometer 10.

The windmill rotor 7 includes blades 8 and a hub 9 supporting the blades8. The blades 8 are provided so that the pitch angle thereof isvariable. More specifically, as shown in FIG. 2, the hub 9 containstherein hydraulic cylinders 11 driving the blades 8 and servo valves 12supplying hydraulic pressure to the hydraulic cylinders 11. Thehydraulic pressure supplied to the hydraulic cylinders 11 is controlledby the openings of the servo valves 12, thereby controlling the blades 8to a desired pitch angle.

FIG. 3 is a block diagram showing details of the configuration of thewind power generator system 1. The wind power generator system 1 in thisembodiment is a sort of doubly-fed variable speed wind turbine system.Namely, the wind power generator system 1 of this embodiment isconfigured to output the power generated by the wound-rotor inductiongenerator 5 to the power grid 13 from both of the stator and rotorwindings. Specifically, the wound-rotor induction generator 5 has thestator winding directly connected to the power grid 13 and the rotorwinding connected to the power grid 13 through an AC-DC-AC converter 17.

The AC-DC-AC converter 17, which includes an active rectifier 14, a DCbus 15 and an inverter 16, converts AC power received from the rotorwinding into AC power adapted to the frequency of the power grid 13. Theactive, rectifier 14 converts the AC power generated by the rotorwinding into DC power and outputs the DC power to the DC bus 15. Theinverter 16 converts the DC power received from the DC bus 15 into ACpower of a frequency equal to that of the power grid 13 and outputs theAC power to the power grid 13. The output power which the wound-rotorinduction generator 5 outputs to the power grid 13 is controlled by theactive rectifier 14 and the inverter 16.

The AC-DC-AC converter 17 also has a function of converting AC powerreceived from the power grid 13 into AC power adapted to the frequencyof the rotor winding, and the AC-DC-AC converter 17 is used to excitethe rotor winding, depending on the operating state of the wind powergenerator system 1. In this case, the inverter 16 converts the AC powerinto the DC power and outputs the DC power to the DC bus 15. The activerectifier 14 converts the DC power received from the DC bus 15 into theAC power adapted to the frequency of the rotor winding and supplies theAC power to the rotor winding of the wound-rotor induction generator 5.

A control system of the wind power generator system 1 includes a PLG(pulse logic generator) 18, a main control unit 19, a voltage/currentsensor 20, a converter drive control unit 21, a pitch control unit 22,and a yaw control unit 23.

The PLG 18 measures the rotational speed ω of the wound-rotor inductiongenerator 5 (hereinafter, referred to as the generator rotational speeda). The voltage/current sensor 20, which is provided on power linesconnected between the wound-rotor induction generator 5 and the powergrid 13, measures the voltage V_(grid) of the power grid 13 (“gridvoltage”) and the output current I_(grid) outputted from the wound-rotorinduction generator 5 to the power grid 13.

The main control unit 19 generates an real power command P*, a reactivepower command Q* and a pitch angle command β*, in response to thegenerator rotational speed ω measured by the PLG 18, and also generatesa yaw command in response to the wind speed and the wind directionmeasured by the anemometer 10. As described later in detail, one featureof the wind power generator system 1 of this embodiment is a controlalgorithm for generating the real power command P* and the pitch anglecommand β*.

The converter drive control unit 21 controls real power P and reactivepower Q outputted to the power grid 13 in response to the real powercommand P* and the reactive power command Q*, respectively. Theconverter drive control unit 21 also controls the turn-on-and-off ofpower transistors within the active rectifier 14 and the inverter 16.Specifically, the converter drive control unit 21 calculates the realpower P and the reactive power Q to be outputted to the power grid 13from the voltage V_(grid) and the output current I_(grid) measured bythe voltage/current sensor 20. Further, the converter drive control unit21 generates PWM signals for the PWM control in response to thedifference between the real power P and the real power command P* andthe difference between the reactive power Q and the reactive powercommand Q*, and supplies the generated PWM signals to the activerectifier 14 and the inverter 16. The real power P and the reactivepower Q outputted to the power grid 13 are thereby controlled.

The pitch control unit 22 controls the pitch angle β of the blades 8 inresponse to the pitch angle command β* transmitted from the main controlunit 19. The pitch angle β of the blades 8 is controlled to coincidewith the pitch angle command β*. The yaw control unit 23 controls thenacelle rotation mechanism 4 in response to the yaw command transmittedfrom the main control unit 19. The nacelle 3 is oriented to thedirection indicated by the yaw command.

An AC/DC converter 24 is connected to the power lines connected betweenthe power grid 13 and the wound-rotor induction generator 5. The AC/DCconverter 24 generates DC power from AC power received from the powergrid 13. The AC/DC converter 24 supplies the DC power to the controlsystem of the wind power generator system 1, particularly to the servovalves 12, the main control unit 19, and the pitch control unit 22 usedto control the pitch angle β of the blades B.

Moreover, the wind power generator system 1 is provided with anuninterruptible power supply system 26 which includes a battery charger27 and an emergency buttery 28 so as to stably supply DC power to theservo valves 12, the main control unit 19, and the pitch control unit22. From requirements of wind power generator system standards, it isnecessary for the wound-rotor induction generator 5 to remain connectedto the power grid 13 even when the grid voltage V_(grid) falls. Thisrequires appropriately controlling the pitch angle of the blades 8 tothereby maintain the rotational speed of the wound-rotor inductiongenerator 5 to a desired value even when the voltage of the power grid13 falls. To satisfy such requirements, when the grid voltage V_(grid)falls to a predetermined voltage, the uninterruptible power supplysystem 26 is connected to the servo valves 12, the main control unit 19,and the pitch control unit 22 by a switch 25, to supply power from theemergency battery 28 to the servo valves 12, the main control unit 19,and the pitch control unit 22. The pitch angle of the blades 8 isthereby kept controlled. The emergency battery 28 is connected to thebattery charger 27. The battery charger 27 charges the emergency battery28 with the DC power supplied from the AC/DC converter 24.

One feature of the wind power generator system 1 of this embodiment isoptimized control of the output power P of the wound-rotor inductiongenerator 5. FIG. 4 is a graph showing the relationship between the realpower command P* and the rotational speed ω of the wound-rotor inductiongenerator 5, depicting a method of controlling the output power Pperformed by the wind power generator system 1 of this embodiment.

When the generator rotational speed ω is smaller than a minimumrotational speed ω_(min), the real power command P* for the wound-rotorinduction generator 5 is controlled to zero. The minimum rotationalspeed ω_(min) is a minimum rotational speed at which power can begenerated by the wound-rotor induction generator 5, and the minimumrotational speed ω_(min), is determined in accordance withcharacteristics of the wind power generator system 1. When the generatorrotational speed ω is larger than the minimum rotational speed ω_(min),the real power command P* is controlled in one control mode selectedfrom two control modes: an optimum curve control mode and a rated valuecontrol mode.

In the optimum curve control mode, the real power command P* iscontrolled to coincide with an optimized power P_(opt) defined by thefollowing Equation (1):

P_(ope)=Kω³,  (1)

where K is a predetermined constant. It is known that it is optimum forthe wind power generator system 1 to control the output power to beproportional to the cube of the generator rotational speed. Accordingly,the output power P is controlled to be proportional to the cube of thegenerator rotational speed ω in the first control mode. The optimumcurve control mode is used mainly in a range in which the generatorrotational speed ω is larger than the minimum rotational speed ω_(min)and smaller than a rated rotational speed ω_(max). Note that the ratedrotational speed ω_(max) is the rotational speed at which thewound-rotor induction generator 5 operates in the steady operation. Thegenerator rotational speed ω is controlled to the rated rotationalspeeds ω_(max) (if possible) by controlling the pitch angle of theblades 8.

In the rated value control mode, on the other hand, the output power Pis controlled to the rated power P_(rated). The rated value control modeis used mainly in a range in which the generator rotational speed ω isequal to or larger than the rated rotational speed ω_(max). In a steadycondition in which wind blows with a rated wind speed, the generatorrotational speed ω is controlled to the rated rotational speed ω_(max)the output power P is controlled to the rated power P_(rated).

An important feature of the wind power generator system 1 of thisembodiment is in a fact that switching from the rated value control modeto the optimum curve control mode is made in response to the pitch angleβ of the blades 8. When the generator rotational speed ω is increased toreach the rated rotational speed ω_(max), the power control isunconditionally switched from the optimum curve control mode to therated value control mode. When the generator rotational speed ω isdecreased below the rated rotational speed ω_(max), on the other hand,the pitch angle β is first reduced. The power control is not switchedfrom the rated value control mode to the optimum curve control modeuntil the pitch angle ω reaches a minimum value β_(min). Namely, thereal power command P* is switched from the rated power P_(rated) to theoptimized power value P_(opt). In other words, the real power command P*is kept at the rated power β_(rated) unless the pitch angle β reachesthe minimum value β_(min) (that is, the pitch angle command β* reachesthe minimum value β_(min)). It should be noted that the fact that thepitch angle β is set to the minimum angle β_(min) implies that theoutput coefficient of the windmill rotor 7 is maximum with the pitchangle β set to the fine-side limit value, since the pitch angle β is theangle formed between the chords of the blades 8 and the rotation planeof the windmill rotor.

The control in which the output power β is kept at the rated powerP_(rated) until the pitch angle β reaches the minimum value β_(min) iseffective for suppressing output power fluctuation and avoidinggeneration efficiency reduction when a transient wind null occurs. Underthe above-described control, the real power command P* is kept at therated power P_(rated) when the generator rotational speed is reducedbelow the rated rotational speed ω_(max) if such state continues onlyfor a short time, and thereby the fluctuation in the output power P issuppressed. Furthermore, the wind power generator system 1 of thisembodiment allows making effective use of rotational energy of thewindmill rotor 7, effectively improving the generation efficiency, sincethe output power P is not reduced from the rated power P_(rated) untilthe increase in the output coefficient of the windmill rotor 7 throughthe reduction in the pitch angle β becomes impossible, when thegenerator rotational speed ω is reduced below the rated rotational speedω_(max).

It should be noted that the power control is switched from the ratedvalue control mode to the optimum curve control mode irrespectively ofthe pitch angle β (or the pitch angle command β*), when the generatorrotational speed ω is reduced below an intermediate rotational speedω_(M)(=(ω_(min)+ω_(max))/2). It is unpreferable for securing the controlstability to maintain the output power P at the rated power P_(rated)when the generator rotational speed ω is excessively small.

FIG. 5 is a block diagram showing an example of a configuration of themain control unit 19 for realizing a control shown in FIG. 4. It shouldbe noted that FIG. 5 shows only one example of the configuration of themain control unit 19; the main control unit 19 may be implemented ashardware, software, or a combination of hardware and software. The maincontrol unit 19 includes a power control module 31 generating the realpower command P* and the reactive power command Q* and a pitch controlmodule 32 generating the pitch angle command β*.

The power control module 31 includes a selector 33, a subtracter 34, aPI controller 35, a power limiter 36, and a power setting calculator 37.On the other hand, the pitch control module 32 includes a subtracter 38,a PI controller 39, a subtracter 40, a PI controller 41, and an adder42. The selector 33, the subtracter 34, the PI controller 35, the powerlimiter 36, the power setting calculator 37, the subtracter 38, the PIcontroller 39, the subtracter 40, the PI controller 41, and the adder 42perform respective calculation steps synchronously with a clock used inthe main control unit 19, and the real power command P*, the reactivepower command Q*, and the pitch angle command β* are thereby generated.

In detail, the selector 33 selects one of the minimum rotational speedω_(min) and the rated rotational speed ω_(max) as a power controlrotational speed command ω_(P)* in response to the generator rotationalspeed ω. More specifically, the selector 33 sets the power controlrotational speed command ω_(P)* to the minimum rotational speed ω_(min),when the generator rotational speed ω is equal to or smaller than theintermediate rotational speed ω_(M), and sets the power controlrotational speed command ω_(P)* to the rated rotational speed ω_(max),when the generator rotational speed ω is larger than the intermediaterotational speed ω_(M).

The subtracter 34 calculates the deviation Δω_(P) by subtracting thepower control rotational speed command ω_(P)* from the generatorrotational speed ω. The PI controller 35 performs PI control in responseto the deviation Δω_(P) to generate the real power command P*. Note thatthe range of the generated real power command P* is limited by an powercommand lower limit P_(min) and an power command upper limit P_(max)supplied from the power limiter 36. Namely, the real power command P* islimited to be equal to or higher than the power command lower limitP_(min) and equal to or lower than power command lower limit P_(max).

The power limiter 36 determines the power command lower limit P_(min)and the power command upper limit P_(max) supplied to the PI controller35 in response to the generator rotational speed c and the pitch anglecommand β*. Further, the power limiter 36 supplies the rated powerP_(rated) to the subtracter 40 of the pitch control module 32. Asdescribed later, the power control shown in FIG. 4 is implemented byappropriately determining the power command lower limit P_(min) and thepower command upper limit P_(max) generated by the power limiter 36 aswell as the power control rotational speed command ω_(P)* determined bythe selector 33.

The power setting calculator 37 generates the reactive power command Q*from the real power command P* generated by the PI controller 35 and apower factor command indicating a power factor of the AC power outputtedfrom the wind power generator system 1, and outputs the real powercommand P* and the reactive power command Q*. As described above, thereal power command P and the reactive power command Q* are used tocontrol the real power P and the reactive power Q outputted from thewind power generator system 1, respectively.

The subtracter 38 of the pitch control module 32 calculate the deviationΔω_(β) by subtracting a pitch control rotational speed command Δω_(β)from the generator rotational speed ω. The pitch control rotationalspeed command of is coincident with the rated rotational speed ω_(max),and therefore the deviation Δω_(β) represents the difference between thegenerator rotational speed (and the rated rotational speed ω_(max).

The PI controller 39 performs PI control in response to the deviationΔω_(β) to generate a pitch angle command baseline value β_(in)*. Thepitch angle command baseline value β_(in)* mainly controls the finallygenerated pitch angle command β*, but the pitch angle command baselinevalue β_(in)* does not always coincide with the pitch angle command β*.The pitch angle command baseline value β_(in)* is determined so that thegenerator rotational speed ω is controlled to the rated rotational speedω_(max).

The subtracter 40 generates a deviation ΔP by subtracting the ratedpower P_(rated) from the real power command P*. The PI controller 41performs the PI control in response to the deviation ΔP to generate acorrection value Δβ*. The adder 42 adds up the pitch angle commandbaseline value β_(in)* and the correction value Δβ* to generate thepitch angle command β*.

The subtracter 40 and the PI controller 41 of the pitch control module32 have a role to prevent the pitch control module 32 from undesirablyinterfering with the power control when the generator rotational speed ωincreases up to the rated rotational speed ω_(max) and the power controlis switched from the optimum curve control mode to the rated valuecontrol mode. The PI controller 39 of the pitch control module 32 isdesigned to adjust the generator rotational speed ω to the ratedrotational speed ω_(max). This may result in that the aerodynamic energyto be extracted as the power is undesirably abandoned. Therefore, inthis embodiment, the PI controller 41 generates the correction value Δβ*in response to the difference between the rated power P_(rated) and thereal power command P*, and the pitch angle command β* is corrected withthe correction value Δβ*. The correction value Δβ* is determined so thatthe pitch angle command β* is smaller than the pitch angle commandbaseline value β_(in)*, i.e., the pitch angle β is set closer to thefine-side limit value, when the real power command P* is lower than therated power command P_(rated), i.e., the deviation ΔP (=P*−P_(rated)) isnegative. Such control allows avoiding the pitch angle β from beingcloser to the feather-side limit value just before the generatorrotational speed ω reaches the rated rotational speed ω_(max). After thegenerator rotational speed ω reaches the rated rotational speed ω_(max),the deviation ΔP becomes zero and the correction value Δβ* becomes zero.

FIG. 6 is a table showing operations performed by the power controlmodule 31 and the pitch control module 32 of the main control unit 19.The operations performed by the power control module 31 and the pitchcontrol module 32 will be described for the following five cases:

Case (1): The generator rotational speed ω is equal to or larger thanthe minimum rotational speed ω_(min) and equal to or smaller than theintermediate rotational speed ω_(M) (=(ω_(min)+ω_(max))/2).

In the case (1), the power control rotational speed command ω_(P)* isset to the minimum rotational speed ω_(min) by the selector 33, and thepower command lower limit P_(min) and the power command upper limitP_(max) are set to zero and P_(opt) (=Kω³), respectively. Besides, thereal power command P* is always set to the power command upper limitP_(max), since the deviation Δω_(P), (=ω−ω_(min)) is positive and thegenerator rotational speed ω is controlled to the rated rotational speedω_(max). The real power command P* is eventually set to the optimizedpower value P_(opt), since the power command upper limit P_(max) isP_(opt). In other words, the power control is set to the optimum curvecontrol mode. In this case, the pitch angle command β* is eventually setto the fine-side limit value, i.e., the minimum pitch angle β_(min),since the pitch control module 32 controls the generator rotationalspeed ω to the rated rotational speed ω_(max).

Case (2): The generator rotational speed ω exceeds the intermediaterotational speed ω_(M), whereby the generator rotational speed ω is setin a range where the generator rotational speed ω is larger than theintermediate rotational speed ω_(M) and smaller than the ratedrotational speed ω_(max).

In the case (2), the power control rotational speed command ω_(P)* isset to the rated rotational speed ω_(max) by the selector 33, and thepower command lower limit P_(min) and the power command upper limitP_(max) are set to P_(opt) and P_(rated), respectively. In this case,the real power command P* is always set to the power command lower limitP_(min), since the deviation Δω_(P) (=ω−ω_(max)) is negative and thegenerator rotational speed ω is controlled to the rated rotational speedω_(max) by the pitch control module 32. The real power command P* iseventually set to the optimized power value P_(opt), since the powercommand lower limit P_(max) is P_(opt). In other words, the powercontrol is set into the optimum curve control mode. The above-describedcorrection of the pitch angle command β* with the correction value Δβ*validly works in the case (2). In the case (2), since the real powercommand P* is lower than the rated power P_(rated), the deviation ΔP isnegative and the correction value Δβ* is therefore negative.Accordingly, the pitch angle command β* is reduced below the pitch anglecommand baseline value β_(in)*, that is, the pitch angle β is set closerto the fine-side lower limit. This allows converting the aerodynamicenergy into the power more effectively.

Case (3): The generator rotational speed ω is equal to or larger thanthe rated rotational speed ω_(max).

In the case (3), the power control rotational speed command ω_(P)* isset to the rated rotational speed ω_(max) by the selector 33, and thepower command lower limit P_(min) and the power command upper limitP_(max) are both set to P_(rated). Therefore, the real power command P*is set to the rated power P_(rated). In other words, the power controlis set into the rated value control mode. On the other hand, the pitchangle command β* is controlled by the PI control so that the generatorrotational speed ω is equal to the rated rotational speed ω_(max).

Case (4): The generator rotational speed ω is reduced below the ratedrotational speed ω_(max), whereby the generator rotational speed ω isset in a range larger than the intermediate rotational speed ω_(M) andsmaller than the rated rotational speed ω_(max), while the pitch angle βdoes not reach the minimum pitch angle β_(min).

In the case (4), the power control rotational speed command ω_(P)* isset to the rated rotational speed ω_(max) by the selector 33.Additionally, the power command lower limit P_(min) is set to a smallerone of the one-operation-step previous real power command P* and thepower command upper limit P_(max) at the current operation step, and thepower command upper limit P_(max) is set to the rated power P_(rated).As a result, the real power command P* is set to the rated powerP_(rated). In other words, the power control is set into the rated valuecontrol mode even when the generator rotational speed ω is reduced belowthe rated rotational speed ω_(max). Whether or not the pitch angle βreaches the minimum pitch angle β_(min) is determined based on whetherthe pitch angle command β* coincides with the minimum pitch angleβ_(min). The pitch angle command β* is controlled by PI control so thatthe generator rotational speed ω is set to the rated rotational speedα_(max). In the case (4), the generator rotational speed ω is smallerthe rated rotational speed ω_(max), and therefore the pitch angle β isreduced by the pitch control module 32.

Case (5): The generator rotational speed ω is reduced below the ratedrotational speed ω_(max), whereby the generator rotational speed ω isset in a range larger than the intermediate rotational speed ω_(M) andsmaller than the rated rotational speed ω_(max), and the pitch angle βreaches the minimum pitch angle β_(min).

In the case (5), the power control rotational speed command ω_(P)* isset to the rated rotational speed ω_(max) by the selector 33, and thepower command lower limit P_(min) and the power command upper limitP_(max) are set to P_(opt) and P_(rated), respectively. In this case,the real power command P* is always set to the power command lower limitP_(min), since the deviation ωΔ_(P) (=ω−ω_(max)) is negative and thegenerator rotational speed ω is controlled to the rated rotational speedω_(max) by the pitch control module 32. The real power command P* iseventually set to the optimized power value P_(opt), since the powercommand lower limit P_(max) is P_(opt). In other words, the powercontrol is switched from the rated value control mode to the optimumcurve control mode.

FIG. 7 is a graph showing an example of the operation performed by thewind power generator system 1 of this embodiment. The real power commandP* is set to the optimized power value P_(opt) until the generatorrotational speed ω reaches the rated rotational speed ω_(max) after thewind power generator system 1 starts operating (the above-described case(2)). Accordingly, the outputted real power P is increased as thegenerator rotational speed ω increases. The pitch angle command β* isset to the minimum pitch angle β_(min) so as to allow the generatorrotational speed ω to reach the rated rotational speed ω_(max).

When the generator rotational speed ω exceeds the rated rotational speedω_(max), the real power command P* is set to the rated power P_(rated)(the above-described case (3)) Accordingly, the outputted real power Pis kept at the rated power P_(rated). Since the generator rotationalspeed ω exceeds the rated rotational speed ω_(max), the pitch anglecommand β* increases and the pitch angle β is varied toward thefeather-side limit value.

When a transitional wind null occurs, the generator rotational speed ωsharply decreases. The pitch control module 32 reduces the pitch anglecommand β* so as to maintain the generator rotational speed ω at therated rotational speed ω_(max) to thereby reduce the pitch angle β, thatis, to vary the pitch angle β toward the fine side. Even when thegenerator rotational speed ω is reduced below the rated rotational speedω_(max), the real power command P* is kept at the rated power P_(rated)as long as the pitch angle β does not reach the minimum pitch angleβ_(min). Therefore, the outputted real power P is also kept at the ratedpower P_(rated).

In the operation shown in FIG. 7, the generator rotational speed ωreturns to the rated rotational speed ω_(max) again before the pitchangle β reaches the minimum pitch angle β_(min), so that the real powerP is kept at the rated power P_(rated). In this way, the wind powergenerator system 1 of this embodiment suppresses the output powerfluctuation when a transient wind null occurs. Furthermore, the windpower generator system 1 of this embodiment makes effective use of therotational energy of the windmill rotor 7 and improves the generationefficiency, since the output power P is not reduced below the ratedpower P_(rated) until the increase in the output coefficient of thewindmill rotor 7 through the reduction in the pitch angle β becomesimpossible, when the generator rotational speed ω is reduced below therated rotational speed ω_(max).

It is preferable that the wind power generator system 1 of thisembodiment is configured to perform various control methods inaccordance with various operating situations. FIG. 8 shows a preferredconfiguration of the wind power generator system 1 configured to performcontrols accordingly to various operating situations.

First, in the wind power generator system 1 shown in FIG. 8, the maincontrol unit 19 detects an occurrence of a gust (rush of wind) by thewind speed and the wind direction measured by the anemometer 10. Themain control unit 19 may detect the occurrence of the gust on the basisof the generator rotational speed ω in place of the wind speed and thewind direction. When the main control unit 19 detects the occurrence ofthe gust, the real power command P* is controlled so as not toexcessively increase the rotational speed of the windmill rotor 7.Specifically, as shown in FIG. 9, when the occurrence of the gust isdetected based on the wind speed and the wind direction (Step S01), theacceleration of the windmill rotor 7 (rotor acceleration) or therotational speed of the windmill rotor 7 (rotor rotational speed) ismonitored. When the rotor acceleration or the rotor rotational speedexceeds a predetermined limit value (Step S02), the real power commandP* is increased (Step S03). When the real power command P is controlledto the rated power P_(rated) until just before the step S03, the realpower command P* is controlled to be increased above the rated powerP_(rated). The rotational energy of the windmill rotor 7 is therebyconverted into electric energy and consumed by the power grid 13. Thisdecelerates the windmill rotor 7.

Moreover, the wind power generator system 1 shown in FIG. 8 isconfigured so that the nacelle rotation mechanism 4 moves the rotationplane of the windmill rotor 7 away from the windward direction tothereby stop the windmill rotor 7, when the pitch control unit 22detects a failure in the pitch drive mechanism that drives the blades 8.To achieve this goal, the wind power generator system 1 shown in FIG. 8is configured so that the pitch control unit 22 is adapted to detect afailure in the hydraulic cylinders 11 and/or the servo valves 12 shownin FIG. 2. The main control unit 19 generates a yaw command in responseto the detection of the failure, when a failure is detected in thehydraulic cylinders 11 and/or the servo valve 12.

FIG. 10 shows a procedure of moving the rotation plane away from thewindward direction. When the pitch control unit 22 detects a failure inthe hydraulic cylinders 11 and/or the servo valves 12 (Step 506), apitch failure signal is activated. In response to the activation of thepitch failure signal, the main control unit 19 controls a yaw angle ofthe nacelle 3, thereby moving the rotation plane of the windmill rotor 7away from the windward direction (step S07). The windward direction canbe determined from the wind direction measured by the anemometer 10. Bymoving the rotation plane of the windmill rotor 7 away from the windwarddirection, the wind speed of wind flowing in the windmill rotor 7 isreduced and the rotational torque is reduced (Step S08). As a result,the windmill rotor 7 is decelerated and stopped.

In addition, the wind power generator system 1 shown in FIG. 8 isconfigured so as to control the reactive power Q supplied to the powergrid 13 when the grid voltage V_(grid) is excessively increased ordecreased, and to perform a pitch control in response to the reactivepower Q. FIG. 11 is a flowchart showing procedures of such control.

When the grid voltage V_(grid) is increased above X % of a predeterminedrated voltage V_(rated) (where X is a predetermined value larger than100) or is reduced below Y % of the predetermined rated voltageV_(rated) (where Y is a predetermined value smaller than 100) (StepS11), the power factor command fed to the power control module 31 ismodified (Step S12). The modified power factor command may be fed from acontrol system of the power grid 13; instead, the main control unit 19may in itself modify the power factor command in accordance with thegrid voltage V_(grid). This results in that the reactive power commandQ* is reduced when the grid voltage V_(grid) exceeds X % of thepredetermined rated voltage V_(rated), and that the reactive powercommand Q* is increased when the grid voltage V_(grid) exceeds Y % ofthe predetermined rated voltage V_(rated). Since the apparent power Ssupplied from the wind power generator system 1 to the power grid 13 isconstant, the real power command P* is increased when the reactive powercommand Q* is reduced, while the real power command P* is reduced whenthe reactive power command Q* is increased. The AC-DC-AC converter 17 iscontrolled in response to the real power command P* and the reactivepower command Q*, thereby controlling the reactive power Q supplied tothe power grid 13 (step S13).

When the reactive power command Q* is largely increased, the real powercommand P* is reduced, and this causes reduction in the output of thewind power generator system 1. To avoid such a problem, the pitch anglecommand β* is reduced (that is, the pitch angle command β* is variedtoward the fine side) when the increase in the reactive power command Q*is larger than a predetermined increase amount, thereby increasing thereal power P (step 515).

When the reactive power command Q* is largely reduced, the real powercommand P* is increased, and this unnecessarily increases the output ofthe wind power generator system 1. To avoid such a problem, the pitchangle command β* is increased (that is, the pitch angle command β* isvaried toward the feature side) when the decrease in the reactive powercommand Q* is larger than a predetermined decrease amount, therebyreducing the real power P.

Furthermore, the wind power generator system 1 shown in FIG. 8 isconfigured so as to increase the outputted real power P while theemergency battery 28 is charged. This addresses compensation for powerused to charge the emergency battery 28. Specifically, as shown in FIG.12, when the battery charger 27 starts charging the emergency battery 28(step S21), the battery charger 27 activates a charge start signal. Themain control unit 19 increases the real power command P* in response tothe activation of the charge start signal (step S22). The increaseamount of the real power command P* is set to be equal to the amount ofthe power used to charge the emergency battery 28. When the emergencybattery 28 is not charged, the real power command P* generated by the PIcontroller 35 is used to control the AC-DC-AC converter 17.

It should be noted that the present invention is not to be interpretedto be limited to the above-stated embodiments. For example, although thewind power generator system 1 of this embodiment is a doubly-fedvariable speed wind turbine system, the present invention is alsoapplicable to other kinds of wind power generator system capable ofvarying both the rotational speed of the windmill rotor and the pitchangle. For example, the present invention is applicable to a wind powergenerator system configured so that an AC-DC-AC converter converts allthe AC power generated by the generator into AC power adapted to thefrequency of the power grid. Further, the emergency battery 28 may becharged not with the power received from the power grid but with thepower outputted from the generator.

Moreover, it is apparent for the person skilled in the art that therotational speed of the windmill rotor 7 may be used in place of thegenerator rotational speed ω, since the rotational speed of the windmillrotor 7 depends on the generator rotational speed ω. For example, as isthe case of this embodiment, the rotational speed of the windmill rotor7 has one-to-one correspondence to the generator rotational speed ω whenthe windmill rotor 7 is connected to the wound-rotor induction generator5 through the gear 6. The rotational speed of the windmill rotor 7 canbe used in place of the generator rotational speed ω, even when acontinuously variable transmission such as a toroidal transmission isused in place of the gear 6; the generator rotational speed ω increasesin accordance with an increase in the rotational speed of the windmillrotor 7.

1. A wind power generator system comprising: a windmill rotor includinga blade whose pitch angle is variable; a generator driven by saidwindmill rotor; and a control unit controlling output power of saidgenerator and said pitch angle of said blade in response to a rotationalspeed of said windmill rotor or said generator, wherein said controlunit performs a first control in which said output power is controlledin accordance with a predetermined power-rotational speed curve untilsaid rotational speed is increased to reach a predetermined ratedrotational speed, and performs a second control in which said outputpower is controlled to a predetermined rated power when said rotationalspeed exceeds said rated rotational speed, and wherein said control unitis responsive to said pitch angle for maintaining a state of performingsaid second control or for switching to a state of performing said firstcontrol, when said rotational speed is reduced below said ratedrotational speed after said control unit is once set to the state ofperforming said second control.
 2. The wind power generator systemaccording to claim 1, wherein, when said rotational speed is reducedbelow said rated rotational speed after said control unit is once set tothe state of performing said second control, said control unit maintainsthe state of performing said second control for a case when said pitchangle is larger than a predetermined pitch angle, and does not switch tothe state of performing said first control until said pitch anglereaches said predetermined pitch angle.
 3. The wind power generatorsystem according to claim 1, wherein said control unit controls saidpitch angle in response to a difference between said rotational speedand a predetermined rated rotational speed and a difference between saidoutput power and said rated power.
 4. The wind power generator systemaccording to claim 3, wherein said control unit controls said pitchangle so as to reduce said pitch angle when said output power is lessthan said rated power.
 5. The wind power generator system according toclaim 1, wherein said control unit increases the output power of saidgenerator in response to said rotational speed when detecting a gust. 6.The wind power generator system according to claim 1, furthercomprising: a rotation mechanism rotating a rotation plane of thewindmill rotor; and a wind direction detector detecting a windwarddirection, wherein said windmill rotor includes a pitch drive mechanismdriving said blade, and wherein, when said control unit detects afailure of said pitch drive mechanism, said control unit control saidrotation mechanism to move the rotation plane of said windmill rotoraway from said windward direction.
 7. The wind power generator systemaccording to claim 1, wherein said control unit is responsive to avoltage of a power grid connected to said generator for controlling areactive power outputted from said generator to said power grid,controlling said pitch angle in response to said reactive power.
 8. Thewind power generator system according to claim 1, further comprising: anemergency battery; and a battery charger charging said emergency batterywith power received from said power grid, wherein said windmill rotorincludes a pitch drive mechanism driving said blade, wherein saidemergency battery supplies power to said pitch drive mechanism and saidcontrol unit when a voltage of the power grid connected to the generatoris decreased, wherein said control unit controls said output power so asto increase said output power while said emergency battery is charged.9. A control method of a wind power generator system including awindmill rotor including a blade whose pitch angle is variable and agenerator driven by said windmill rotor, said control method comprising:a control step of controlling output power of said generator and saidpitch angle of said blade in response to a rotational speed of saidwindmill rotor or said generator, wherein said control step includes:(A) a step of performing a first control in which said output power iscontrolled in accordance with a predetermined power-rotational speedcurve, until said rotational speed is increased to reach a predeterminedrated rotational speed; (B) a step of performing a second control inwhich said output power is controlled to a predetermine rated power whensaid rotational speed exceeds said rated rotational speed; and (C) inresponse to said pitch angle, maintaining a state of performing saidsecond control or switching to a state of performing said first control,when said rotational speed is reduced below said rated rotational speedafter the state of performing said second control is once established.